| Strict controls are in place to ensure that high standards of navigation, dredging, berthing and anchoring can be maintained for safety and operational reasons. The design of the immersed tube units has required careful consideration of these constraints. Dredging of the trench in the river bed is expected to take up to four months and can only take place between November and March when the number of migratory fish such as salmon and sea trout are at their natural lowest. Dredging would cause a minor obstruction to river traffic. The laying of each unit is expected to take approximately one day, and this would necessitate the closure of the river to all river traffic. To minimise disruption, unit laying would not occur on consecutive days. After the units have been put into position in the trench, a further three months' work would be required in the river occasioning further, minor, obstructions to traffic. Once the tunnel is operational, there would be no effect on navigation. The depth of the new tunnel below the river bed would be to a depth agreed with the Port of Tyne Authority to accommodate any possible future development. River bed sediments would be disturbed during excavation of the trench across the river, bedding the immersed tube sections and backfilling the excavation. Some sediments would be suspended in the flowing water above the river bed, and could be carried away from the construction area. In turn, some of this sediment would be redeposited away from the trench. Computer modelling has demonstrated that these effects would be both reversible and very localised (mainly within a radius of about 50 metres). The Environment Agency has specified maximum allowable values for the concentration of suspended sediment disturbed during construction. Through a modelling study, it has been confirmed that this target would be achieved under almost all foreseeable construction and river flow scenarios. |
